An exhaust gas temperature sensor is used, for example, for an exhaust gas purifying system for purifying NOx included in exhaust gas emitted from an engine. In the system, in order to add a proper amount of a reducing agent to the exhaust gas, the addition amount of the reducing agent is increased/decreased according to exhaust gas temperature detected by an exhaust gas temperature sensor. However, when abnormality such as a failure occurs in the exhaust gas temperature sensor, the proper amount of the reducing agent may not be added to the exhaust gas. To address the problem, an abnormality detecting apparatus for detecting abnormality in the exhaust gas temperature sensor is provided.
The exhaust gas temperature sensor is roughly classified into a thermister type and a platinum-resistance type. An exhaust gas temperature sensor of the thermister type has an output characteristic that the resistance value (output voltage) decreases nonlinearly as the temperature rises. Namely, when the exhaust gas temperature is low, the resistance value is high, and when the exhaust gas temperature is high, the resistance value is low. When abnormality occurs in the exhaust gas temperature sensor, for example, in the case of sensor short-circuit, the resistance value indicates a value in a high-temperature range. In the case of disconnection, the resistance value indicates a value in a low-temperature range.
In the case of sensor short-circuit, an output of the exhaust gas temperature sensor is largely lower than a resistance value which can be output in a normal state, thus the sensor short-circuit can be detected. However, in the case of disconnection, an output of the exhaust gas temperature sensor is similar to the output characteristic at the low temperature, thus it is difficult to detect the disconnection.
In conventional techniques, although short-circuit and disconnection of an exhaust gas temperature sensor can be detected, since abnormality is detected using a threshold defining a normal range and an abnormal range, abnormality which occurs in the normal range may not be detected. In this case, for example, abnormality can be supposed such that, an output of the exhaust gas temperature sensor changes according to the exhaust temperature rise, however, when the exhaust gas temperature rises to a certain temperature, the output thereof does not change and is fixed to an almost constant value for some reason at the certain exhaust gas temperature.
Therefore, focused on the fact that exhaust gas temperature rises immediately after start of the engine, an apparatus for detecting abnormality in an exhaust gas temperature sensor is needed to at least detect occurrence of disconnection when the exhaust gas temperature sensor cannot detect an output of a predetermined value or larger.